In recent years, crystalline propylene-α-olefin copolymers, composed mainly of propylene and typically also containing ethylene and/or 1-butene as comonomer, have been widely used for film applications on account of their characteristics of good transparency, excellent impact properties and good heat-sealability, etc. Metallised films obtained by depositing metals onto plastic films in vacuo have been widely used for many years eg for packaging films, with the substrate onto which the metal is deposited often being the aforementioned propylene-α-olefin copolymers. In particular, single layer cast films or biaxially oriented polypropylene (BOPP) films, either single or multilayered, which have a metal coating deposited on the surface, are commonly used for food packaging. Such single layer cast films BOPP films can also be used in similar non-metallised applications.
One problem with propylene-α-olefin copolymers used in such applications is their low flexibility and also their stickiness at relatively high temperature and particularly at high processing speeds. This can cause problems during processing, which is carried out at high temperature and involves stretching of the film followed by metallisation. Adhesion of the propylene polymer to the rollers in the processing line can cause wrinkles and protuberences to appear on the film during the stretching process. This can result in films of poor optical quality and appearance, particularly after metallisation. Such problems are typically addressed by the addition to the propylene-α-olefin copolymers of slip agents, which reduce the risk of the polymer sticking to the rollers. Examples of such slip agents include fatty acid amides or waxes. However the use of such compounds provides its own set of problems. For example they can cause a reduction in surface tension of the film which makes printing or adhesion onto a metallized surface much more difficult. Use of some waxes can also impart poor organoleptic properties to the film.
JP 2297435A discloses a metallised film in which the metal is deposited on a layer comprising 18-60 wt % of a straight chain polyethylene having a density of 925-937 kg/m3 and 40-82 wt % of a polypropylene. This is said to improve adhesion of the metal film.
GB 2125802A attempts to overcome the disadvantages associated with known slip agents by providing a blend of 80-96 wt % of a crystalline propylene-α-olefin copolymer containing 70% by weight or more of propylene and 4-20 wt % of a high density polyethylene having a density of 940 kg/m3 or more. GB 2125802A states that if the density of the high density polyethylene is below 940 kg/m3, it is impossible to obtain metallised films having a good “taken-up figure”, that is to say a film having a surface free of surface defects such as wrinkles or protuberences.